Recently, with the increased interest in developing a new standard for transmitting high definition television signals, there have been several proposals to include supplemental video information modulated on a subcarrier that is in quadrature with the picture carrier. One of these exemplary systems is described in a paper by M. A. Isnardi et al. entitled "Decoding Issues In The ACTV System", IEEE Trans. on Consumer Electronics, Vol. 34, No. 1, 2/88, pp. 111-120, and another is described in a paper by Y. Yasumoto et al. entitled "A Wide Aspect Ratio Television System With Full NTSC Compatibility", IEEE Trans. on Consumer Electronics, Vol. 34, No. 1, 2/88, pp. 121-127.
FIG. 1, labeled "prior art" is a block diagram showing an exemplary video signal transmission and reception system in which information is modulated in quadrature with the picture carrier signal. The system shown in FIG. 1 is substantially the same as that shown in U.S. Pat. No. 4,882,614, entitled MULTIPLEX SIGNAL PROCESSING APPARATUS, which is hereby incorporated by reference. In the system shown in FIG. 1, a main video signal S1(t) is modulated, by a modulator 114, onto a picture carrier signal 2COS(2.pi.f.sub.o t) provided by a source 118. The signal S1(t) may be, for example, a standard NTSC composite video signal. The double sideband signal provided by the modulator 114 is filtered by a vestigial sideband (VSB) filter 121. The filter 121 produces a VSB signal having a double sideband portion, occupying a band of frequencies approximately 750 KHz below and above the picture carrier signal, and a single sideband portion, occupying a band of frequencies between approximately 750 KHz and 4.2 MHz above the picture carrier. An exemplary frequency response Characteristic for the VSB filter 121 is shown in FIG. 3c.
A second modulator 116 modulates a carrier signal 2SIN(2.pi.f.sub.o t) with a supplementary video signal S2(t) The carrier signal 2SIN(2.pi.f.sub.o t) is in quadrature (i.e., shifted in phase by 90.degree.) with respect to the picture carrier signal. The supplementary video signal may include, for example, information on relatively high frequency signal components of a high-definition television signal, or information to be used to convert the aspect ratio of the video signal from the standard 4:3 to a wide screen 16:9.
In the exemplary system shown in FIG. 1, the double sideband modulated supplementary signal developed by the modulator 116 is applied to an inverse Nyquist filter 122. An inverse Nyquist filter has a response which is the complex conjugate (about the picture carrier ) of the Nyquist filter response of most television receivers. The filter 122, which may have the frequency response characteristic shown in FIG. 3a, produces an output signal which is added, by signal summing circuitry 124, to the signal provided by the VSB filter 121.
The signal provided by the summing circuitry 124 is the output signal of the transmitter. This signal is sent to the viewer through a multipath channel which includes both a direct signal propagation path and reflecting paths which generate multipath distortion in the received video signal.
When the signal is received by the tuner (not shown) of a television receiver, it is applied to an intermediate frequency (IF) amplifier 127. In the exemplary receiver shown in FIG. 1, the IF amplifier 127 includes two filters, a Nyquist filter 128 and a band-pass filter 129. The Nyquist filter 128 converts the received VSB signal into a signal which, when demodulated, produces a baseband signal having no substantial attenuation in its frequency spectrum from 0 MHz to 4.2 MHz.
In the exemplary high-definition television receiver shown in FIG. 1, the Nyquist filter 128 also reduces the amplitude of the modulated supplementary signal S2(t). Furthermore, if the Nyquist filter 128 is matched to the inverse Nyquist filter 122 in the transmitter, the filter 128 substantially reduces crosstalk of the quadrature signal into the in-phase signal by making the sidebands of the modulated quadrature signal symmetric about the picture carrier.
The band-pass filter 129 passes only the double sideband portion of the in-phase signal and the inverse-Nyquist filtered double sideband quadrature signal. The signals provided by the filters 128 and 129 of the IF amplifier 127 are applied to respective synchronous demodulators 130 and 132.
The demodulators 130 and 132 multiply the output signals provided by the Nyquist filter 128 and the band pass filter 129 by the recovered in-phase picture carrier signal RC.sub.i and quadrature picture carrier signal RC.sub.q, respectively. The signals produced by the demodulators 130 and 132 are filtered by the respective low pass filters 138 and 140 to produce in-phase and quadrature phase baseband signals R.sub.i and R.sub.q, respectively. The signals R.sub.i and R.sub.q are processed by a ghost reduction filter 142, to produce the recovered baseband signals S1'(t) and S2'(t).
It has been known for many years that systems for canceling multipath distortion in standard television signals (i.e. ghost cancellation systems) work best if they operate on both the in-phase and quadrature phase components of the received video signals. A reflecting object which gives rise to multipath distortion may add a delayed and attenuated version of the quadrature phase component into the in-phase component of a video signal. The amount by which the phase of a ghost signal is shifted with respect to the original signal determines the relative proportions of in-phase and quadrature phase signal in the ghost signal.
Exemplary systems for correcting multipath distortion by using a complex filter operating on both the in-phase video and quadrature phase video signals are disclosed in U.S. Pat. Nos. 4,703,357 to Ng et al and 4,864,403 to Chao et al., which are hereby incorporated by reference. The signals being processed by these systems include meaningful information only in the in-phase component of the video signal. The quadrature phase component exists only because standard television signals are transmitted using vestigial sideband modulation and thus have unequal sidebands relative to the picture carrier.
As shown in FIG. 1, the in-phase carrier signal RC.sub.i is recovered from the received modulated video signal by picture carrier extraction circuitry 123. This signal is then shifted 90.degree. in phase by phase shifter 125 to generate the quadrature phase carrier signal RC.sub.q. In the absence of significant multipath distortion, the recovered carriers RC.sub.i and RC.sub.q are at substantially the same frequency and phase as the respective carrier signals 2COS(2.pi.f.sub.o t) and 2SIN(2.pi.f.sub.o t) that were modulated to generate the transmitted video signal. When, however, a significant ghost signal distorts the received video signal, the recovered carrier signals RC.sub.i and RC.sub.q may be shifted in phase significantly with respect to the original carrier signals. The difference between the recovered carrier signals and the original carrier signals occurs because the carrier extracted by the receiver is the vector sum of the main carrier signal and the ghost carrier signals. When a ghost carrier signal which is shifted in phase with respect to the main carrier signal has a significant amplitude, there may be a substantial difference between the carrier signals derived from this vector sum and the transmitted carrier signals. This difference in phase may produce significant crosstalk distortion between the detected in-phase and quadrature phase signals.
As set forth above, to avoid crosstalk between the in-phase and quadrature phase signals, the receiver shown in FIG. 1 includes separate IF filters, and presumably separate IF amplifiers. Even with these separate filters there may, be crosstalk of the quadrature phase signal into the in-phase signal if there is a mismatch between the respective slopes and/or breakpoints of the inverse Nyquist filter 122 used in the transmitter and the Nyquist filter 128 used in the receiver.
The use of a separate IF filter and IF amplifier for the supplementary signal adds cost to the receiver and complicates its design. The design complications arise because the signal propagation paths of the signals S1'(t) and S2'(t) are different. Thus, the group delay characteristics of the two filters must either be matched or compensated in other circuitry to ensure that the images produced by the signals S1'(t) and S2'(t) are properly aligned on the display.